Matrix acidizing in carbonate formations is used to improve the production from a well by creating wormholes. The flow and reaction of hydrochloric acid (HCl) in carbonate porous media results in the formation of highly conductive flow channels or wormholes. Wormholes form because of the natural heterogeneity of the porous matrix and the rapid, mass transfer limited, and almost complete dissolution of the mineral in the acid. The acid preferentially flows to the regions of the highest permeability. These initial flow paths are enlarged by rapid dissolution of the matrix material, causing these regions to receive even more of the flow. A dominant channel quickly forms and continues to propagate while diverting flow from other regions. Once formed, the wormhole channels provide negligible resistance to flow and carry essentially all the injection fluid. HCl has also been used as an acidizing treatment to remove near-wellbore damage.
HCl treatment often requires a low injection rate to prevent fracturing the formation rock. In addition, the injection of HCl into carbonate formations at low injection rates results in face dissolution or complete dissolution of the carbonate matrix near the wellbore and causes corrosion. This face dissolution consumes large volumes of acid and provides negligible increases in the conductivity of the formation.
Ethylenediaminetetraaceticacid (EDTA) has been used to stimulate carbonate porous media and remove calcium carbonate scale from underground formations. EDTA is a chelating agent that stimulates by means of sequestering the metal components of the carbonate matrix. The dissolution mechanism is different than that of HCl in that hydrogen ions are not required.
In oilfield chemical treatments, chelating agents are frequently added to stimulation acids to prevent precipitation of solids as the acid spends on the formation are being treated. These precipitates include iron hydroxide and iron sulfide. In addition, chelating agents are used as components in many scale removal/prevention formulations. Chelating formulations based on EDTA, nitriloacetic acid (NTA) and diethylenetriaminepentaacetic acid (DTPA) have been used to control iron precipitation and to remove scale. However, EDTA has low solubility in HCl and is not readily biodegradable. NTA is somewhat better in acid solubility and biodegradability, but has a lower stability constant for iron than EDTA and DTPA and is considered to be an animal carcinogen.